The present invention relates to a vehicle rechargeable battery controller and a method for controlling output of a vehicle rechargeable battery.
In recent years, a hybrid electric vehicle (HEV) having an engine and a drive motor as its power sources has been commercialized. A rechargeable battery is mounted on the HEV and supplies power to the drive motor.
When the engine outputs more power than necessary to drive the vehicle, the HEV drives a power generator with the excessive power and charges the rechargeable battery. When the vehicle is braking or decelerating, the HEV drives a motor with the rotating wheels to charge the rechargeable battery with the motor. In this case, the motor is used as a power generator. When the engine outputs insufficient power, the HEV compensates for the lack of power by discharging the rechargeable battery and driving the motor. In this case, the motor is used as a power source for the HEV.
The HEV accumulates energy in the rechargeable battery. Conventional automobiles discharge such energy into the atmosphere as heat. Thus, the energy efficiency of the HEV is higher than the energy efficiency of conventional automobiles. Further, the HEV drastically improves fuel efficiency as compared with conventional automobiles.
In an HEV, the rechargeable battery for supplying power to the drive motor is also used to supply power to a starter motor for starting the engine. However, the discharge voltage of the rechargeable battery greatly decreases at low temperatures of 0° C. or less. At such low temperatures the rechargeable battery may not be able to start the engine. To solve this problem, Japanese Laid-Open Patent Publication No. 2002-195138 describes a rechargeable battery controller (battery ECU) that limits the discharge power of the rechargeable battery to ensure starting of the engine at low temperatures.
More specifically, the battery ECU sets a minimum voltage at which driving of the starter motor with the rechargeable battery is enabled. The discharge voltage of the rechargeable battery is greater than or equal to the minimum voltage when the starter motor is driven. When the discharge voltage of the rechargeable battery decreases to the minimum voltage after the starter motor is driven, the battery ECU controls the rechargeable battery to temporarily stop the supply of power to the starter motor. The battery ECU waits until discharge current generates heat in the battery and accordingly increases the discharge voltage. The battery ECU then permits the rechargeable battery to resume power supply to the starter motor. The battery ECU continues to execute this control until the engine is started. In this manner, the battery ECU of the prior art controls the rechargeable battery to repeatedly start and stop discharging to ensure that the engine can be started under low temperatures.